Filtering devices allow for direct selection or suppression of frequency components of electrical signals. Those having ordinary skill in the art will appreciate that, for any particular signal, techniques exist which allow the signal to be approximated by a weighted sum of periodic signals (e.g., sine waves and/or cosine waves which repeat themselves within defined time periods). Each periodic signal in the sum has a certain frequency (inversely related to the time period required for the signal to repeat). The weighted periodic signals which are summed to approximate the analog signal may be referred to as the “frequency components” of the signal. One type of filtering device is known in the art as a “graphic equalizer,” since it graphically illustrates the selection or suppression of the various frequency components of a signal.
With reference to the figures, and with reference now to FIG. 1, shows a front elevational view of a control panel of a conventional graphic equalizer 100. Graphic equalizer 100 typically incorporates a plurality of filters (passive or active, digital or analog) which amplify or attenuate electrical signals within discrete frequency passbands. Typically, such equalizers have each filter operated by a slider control related to each discrete passband. Graphic equalizer 100 operates on three frequency bands which are illustrated on a control panel face of graphic equalizer 100. The frequency bands upon which graphic equalizer 100 operates have center frequencies, Frequency A, Frequency B, and Frequency C. Controls 101, associated with each frequency band, allow an operator (e.g., a human operator) to boost (Le., amplify) the frequency band by up to 12 dB or cut (Le., attenuate) the frequency band by upto −12 dB. Depicted, for sake of illustration, is that control 101 of Frequency A is set to a 2 dB boost, control 101 of Frequency B is set to a −2 dB boost, and control 101 of Frequency C is set to a 6 dB boost.
The frequency specific cutting or boosting performed by graphic equalizer 100 is typically achieved by filters centered on frequencies A, B, and C. Those having ordinary skill in the art will recognize that, ideally, each filter would uniformly cut or boost the components of the input signal which exist within passband (eg., 102, 104 and 106 of FIG. 2) of each filter.
FIG. 2 graphically shows magnitude responses of theoretical ideal filters which would preferably be used in conjunction with graphic equalizer 100. Each ideal filter provides a uniform response. A uniform response means that (a) the leftmost and rightmost edges 108, 110 of each passband (a band of frequencies which a filter is designed to let through, or “pass”) are substantially vertical at or near the passband cutoff frequencies 116 defining the passband of each filter, and (b) the maximum amplitude 112 is substantially constant or flat in each passband so as to form sharp corner frequency response 114 therebetween. Those skilled in the art will appreciate that, unfortunately, physically realizable filters do not tend to provide the desired response of the “ideal” filters shown in FIG. 2.
FIG. 3 illustrates magnitude responses which are more representative of physically realizable, as opposed to ideal, filters. Rather than having sharp corner dropoff at the cutoff frequency 116 of each filter's passband 102, 104, 106, physically realizable filters tend to roll off gently rather than have sharp “corner” frequency responses (e.g., 114 of FIG. 2). The fact that physically realizable filters do not provide sharp cutoff allows the energy from one frequency band of the graphic equalizer to bleed into the other frequency bands of the graphic equalizer. As can be seen, such interference tends to be additive, and thus gives rise to a resultant aggregate frequency response 120 which is not at all in keeping with the desired frequency response.
One known desired theoretical solution to the foregoing noted interference problem of FIG. 3 is to manipulate the filtering so as to subtract out the bleeding of the respective filters beyond the cutoff frequencies 116 defining their respective passbands 102, 104, 106. However, as will be appreciated by those having ordinary skill in the art, both the behavior of the individual filters as well as the interactions between filter bleeds, tends to be highly nonlinear and/or unpredictable. The effect of this is that it is extremely difficult to use known techniques to alleviate the interference problems, so in practice the desired theoretical approach is typically not achieved.
One example of how interference problems, such as those illustrated and described in relation to FIG. 3, have previously been addressed in the prior art is disclosed in U.S. Pat. No. 5,687,104 to Lane et al. (hereinafter Lane). Lane teaches generating a unique decoupling matrix by exciting a graphic equalizer using a series of test input vectors applied to a series of pre-stored decoupling matrices, and selecting as the decoupling matrix that matrix which generates the least overall error in graphic equalizer output. Thereafter, user specified graphic equalizer cut-boost input levels are subjected to the selected decoupling matrix to create corrected inputs. The graphic equalizer is then internally set to have these corrected inputs and allowed to operate.
One drawback of the method disclosed by Lane, and other related-art techniques, is that Lane tends to work with fixed Q value filters. (A Q value is a number roughly indicating how well a “real world” filter approaches that of a theoretically ideal filter, such as how “sharp” the corner frequency response 114 will be.) Lane's, and other related-art techniques, also do not recognize that non-linearity of interactions between filters having fixed Q values varies dependent upon selected cut-boost levels. Accordingly, Lane's, and other related art techniques, do not show or suggest alleviating the non-linearity of interactions between filters by constructing filters having “linearizing” Q values which tend to linearize the interactions between filters. Accordingly, a need exists for a graphic equalizing filter system which utilizes linear techniques on filters having linearizing Q values.